A Tier 5 (Your Mobility Scheme) visa cannot be extended, so your friend's permission to remain in the UK will expire on March 14. If he overstays for a few weeks it is unlikely that it will have any immediate consequences (as in, police won't round him up and remove him by force, and he won't be fined when he eventually laves) -- but it will make it much harder for him to be allowed into the UK at a later time.
Being a non-visa national doesn't mean that he automatically has the right to stay in the country -- merely that he gets to apply for "leave to enter" as a visitor at the border rather than in advance at a UK consulate. He cannot make such an application while inside the UK, so the legal minimum he'll need to do is to channel hop.
However, then he will need to convince the Immigration Officer at the border that he's a "genuine visitor" rather than someone who is effectively living in the UK. This will be very hard to do if he tries to enter again immediately after a youth mobility visa expired, so there's a large risk that he will be denied entry.
There are no fixed rules for how long he needs to stay away -- the IO is allowed to believe his story and let him in even if he returns immediately; they just don't have to. Generally the longer he stays outside, the easier will it be to convince an IO that he's not attempting to live in the UK.
I would recommend being back in Japan for at least several months before trying to visit, unless he has a really good explanation for why he's returning as a temporary visitor so soon. "I want to hang out with my friends again" probably won't cut it; "a friend I made here is getting married" might.